Is that boiler a closed combustion device, i.e., have an air intake as well? If so, you probably don't want it there. The instruction manual will have the appropriate restrictions. Since a vent may not seal perfectly, even though it is an exit, there are restrictions on potential co intake.

Modcons don't like intake air with stuff like potentially chlorine, lint, or other chemicals in it, either. Those could be coming out of the vent. Again, your best thing is the installation manual and potentially, the local inspector's office, but they usually go by the restrictions in the installation manual.

All of the guides I've read, show diagrams of the minimum clearances required to things like overhangs, windows, doors, walkways, and other vents. If you are adhering to those, then you should not have problems. When I had my mod-con installed, where it was the easiest to run the vent lines would have been within a few feet of two dryer vents and another heater's exhaust, so I had to run it out the opposite wall of the house. Still within the limits of length, but much more of a pain.